4 TECHNICAL DATA
Table 4.13: Definition of distance code increasing
No. absolute position [mm] distance to previous [periods] distance to previous [mm]
1 1.8 - -
2 3.3 75 1.50
3 4.82 76 1.52
4 6.36 77 1.54
5 7.92 78 1.56
6 9.50 79 1.58
7 11.10 80 1.60
and so on
Distance coded increasing: The reference track comprises several reference marks with increas-
ing neighbour distance distributed over the entire length of the scale. This is one of the most
simple types of distance codes (figure 4.18). Here the distance between two neighbours will
increase with each number by a specific value. Hence, a referencing algorithm will need to
move the encoder across two neighbouring reference marks. From the distance between
the marks, the number of the detected marks and hence the absolute position can be gath-
ered instantly, because each neighbour-distance is unique. There is no need to move across
the entire length of the scale for referencing. A disatvantage of this code is however, that for
very long scales the increasing distance might lead to very large distances, so that very far
referencing movements will be needed if the initial position is somewhere at the upper end
of the scale.
Distance coded alternating: This type of code comprises two types of reference marks, the ones
with even numbers and the ones with odd numbers (see figure 4.19). The distance between
the odd numbers is always the same, while the distance between the even numbers is also
always the same, but has a different value. In conclusion the distance between two neigh-
bours (an odd with an even number) will vary and will also be unique over the entire length
of the scale. The advantage of this code is, that there is no larg gap between two neighbours
even for very long scales up to several meters.
Figure 4.19: Schematic illustration of alternating distance code
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